101. 对称二叉树

 方法一：递归

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root == null) return true;
        return isSame(root.left,root.right);
    }

    public boolean isSame(TreeNode l, TreeNode r) {
        if(l == null && r == null) return true;
        if(l == null || r == null) return false;
        if(l.val != r.val) return false;
        return isSame(l.left,r.right) && isSame(l.right,r.left);
    }

}

方法二：迭代

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
   
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root.left);
        queue.offer(root.right);

        while (!queue.isEmpty()) {
            TreeNode node1 = queue.poll();
            TreeNode node2 = queue.poll();
            if (node1 == null && node2 == null) {
                continue;
            }
            if (node1 == null || node2 == null || node1.val != node2.val) {
                return false;
            }
            queue.offer(node1.left);
            queue.offer(node2.right);
            queue.offer(node1.right);
            queue.offer(node2.left);
        }

        return true;
    }
}